1. Filed of the Invention
The present invention relates to a laser interferometer system. Particularly, the invention relates to a laser interferometer system of high stability without periodic nonlinearity. More particularly, the invention relates to a laser interferometer system for both displacement and straightness measurements. The main applications of the system are in the fields of scientific research, precision engineering, nanometrology, and characterization of MEMS.
2. Description of the Prior Art
Laser interferometry is nowadays widely used in length-relate measurements in precision engineering, metrology, and lithography applications, as well as in advanced scientific applications. The typical prior art for heterodyne interferometry is schematically shown in FIG. 1, see for example, Bagley et al, U.S. Pat. No. 3,656,853, issued Apr. 18, 1972; Sommargren et al, U.S. Pat. No. 4,688,940, issued Aug. 25, 1987. The output laser beam consists of a pair of nominally orthogonal, linearly polarized beams having different frequencies. The properties of having two frequencies and two polarizations in a coaxial beam simplify the design for interferometer unit. However, it leads to produce the periodic non-linearity caused by the mixings of both frequency and polarization, which limits the accuracy of displacement measurements at the nanometer level.
The periodic non-linearity of heterodyne interferometry was first studied by Quenelle, Hewlett Packard J. Vol. 34, pp. 10 (1983), who predicted the worst case error for measurements made using the HP laser interferometer to be about 5 nm. Direct experimental verification was obtained by Sutton, J. Phys. E: Sci. Instrum. Vol. 20, pp. 1290-1292 (1987). A prior art showing that the first-order non-linearity could be compensated for a heterodyne interferometer was reported by Hou and Wilkening, Precision Eng. Vol. 14, pp.91-98 (1992) and by Wilkening et al, U.S. Pat. No. 5,331,400, issued Jul. 19, 1994; the second-order non-linearity could not be compensated by the approach is a major drawback.
For measuring the straightness error, a typical prior art is shown in FIG. 2 (for example as manufactured by Hewlett-Packard and described in U.S Pat. No. 3,790,284), in which a Wollaston prism is used to split the incident input beam traveling along the principal axis into two secondary beams, each one with a different frequency and polarization. The two secondary beams, deviating from each other at an angle alpha, travel to a straightness reflector. The two returned beams from the straightness reflector are back along their incoming paths, respectively, and then make interference in the Wollaston prism along the principal axis. This system has certain disadvantages. First, it is not a symmetrical design for the Wollaston is quite different from the straightness reflector in shape, material, and other properties. Second, the angle from the straightness reflector is hard to match the divergent angle alpha of the two secondary beams. Third, mixings from both the frequency and polarization are not avoided, resulting in periodic nonlinearity.
Other prior art systems, such as those disclosed in U.S. Pat. Nos. 4,787,747, 5,026,163, and 5,757,491, are complex and asymmetric in their interferometer set-ups, so that most of them are not able to be commercialized.
An object of the present invention is to provide an interferometer system having a high stability with no periodic nonlinearity while capable of providing both displacement and straightness measurements. The laser interferometer system bases upon three design principles, the heterodyne frequency, the avoiding mixing, and the perfect symmetry.
In accordance with an embodiment of the present invention, a laser interferometer system capable of measuring accurately both changes in optical path length and changes in displacement is disclosed which comprises: (1) a light source of a single stabilized frequency input beam, preferably a linearly polarized laser output; (2) frequency-shifted means, preferably two acousto-optic modulators, for converting the input beam into a pair of spatial-separated beams having different optical frequencies; (3) splitting means, preferably a pair of energy beam splitters, for converting one of the spatial-separated beams into a pair of measurement beams and the other into a pair of reference beams; (4) a measurement branch, preferably including a beam splitter, a quarter wave plate, a fixed target, and a moving target, for redirecting the measurement beams and then ready for further interfering with other beams; (5) interferometric means, preferably a beam combiner, for making interference of measurement beams and reference beams so as to formn a pair of interferometric measurement beams and a pair of interferometric reference beams; (6) receiving means, preferably a pair of photo-receivers, for converting the interferometric measurement beams and the interferometric reference beams into an interferometric measurement signal and an interferometric reference signal respectively; and (7) accumulating means, preferably a phase comparator/accumulator, for providing an output signal based on the difference in phase between the reference signal and the measurement signal.
In accordance with another embodiment of the present invention, a laser interferometer system capable of measuring accurately both changes in optical path length and in transverse displacement is disclosed which comprises: (1) a light source of a single stabilized frequency input beam, preferably a linearly polarized laser output; (2) frequency-shifted means, preferably two acousto-optic modulators, for converting the input beam into a pair of spatial-separated beams having different optical frequencies; (3) splitting means, preferably a pair of energy beam splitters, for converting one of the spatial-separated beams into a pair of measurement beams and the other into a pair of reference beams; (4) a measurement branch, preferably including a beam splitter, a quarter wave plate, a straightness prism for converging and then diverging the measurement beams, and a straightness reflector having the same shape as the straightness prism but with high reflection coating on the far-end side for re-converging the diverging beams to parallelism and then reflecting back the beams along their incoming paths; (5) interferometric means, preferably a beam combiner, for making interference of measurement beams and reference beams so as to form a pair of interferometric measurement beams and a pair of interferometric reference beams; (6) receiving means, preferably a pair of photo-receivers, for converting the interferometric measurement beams and the interferometric reference beams into an interferometric measurement signal and an interferometric reference signal respectively; and (7) accumulating means, preferably a phase comparator/accumulator, for providing an output signal based on the difference in phase between the reference signal and the measurement signal.
In accordance with the present invention, the laser interferometer systems is capable of measuring accurately changes in optical path length in both displacement and transverse displacement with no periodic nonlinearity through the following steps of method: (1) producing a stabilized single optical input beam from a light source; (2) producing a pair of spatial-separated beams of which the optical properties are the same but with respect to frequency (i.e. heterodyne frequency), in which there is no frequency cross talk to each other (i.e. avoiding mixing); (3) producing a pair of measurement beams from one of the spatial-separated beams having identical optical properties and no polarization cross talk to each other (i.e. avoiding mixing) for both measurement and reference targets or straightness prism and reflector (i.e. perfect symmetry); (4)producing a pair of reference beams from the other of the spatial-separated beams having identical optical properties and no polarization cross talk to each other (i.e. avoiding mixing); (5) interfering the measurement beams with the reference beams, separatively, to form two interferometric reference beams and two interferometric measurement beams in a perfect symmetrical manner; (6) producing a pair of electrically interferometric signals by means of coupling photo-receiver means to receive an interferometric measurement beam and an interferometric reference beam; and (7) producing an output signal, which is proportional to the changes in optical path length between the interferometric measurement signal and the interferometric reference signal, the output signal having substantially eliminated the periodic non-linearity by means of avoiding the frequency cross talk and polarization cross talk on the interferometric measurement and reference beams.
The advantages and features of this invention can be easily comprehended by persons skilled in the art through the drawings and detailed explanations.